The compound CoCl3·4NH3 is an octahedral complex in which NH3 behaves as a strong-field ligand. One mole of its aqueous solution is treated with…

The compound with composition $\mathrm{CoCl_3\cdot4NH_3}$ is $[\mathrm{Co(NH_3)_4Cl_2}]\mathrm{Cl}$: coordination number 6, with two $\mathrm{Cl^-}$ inside the sphere and one ionisable $\mathrm{Cl^-}$, so only 1 mol of AgCl precipitates immediately. Cobalt is $\mathrm{Co^{3+}}$, $3d^6$; with strong-field $\mathrm{NH_3}$ the configuration is $t_{2g}^6e_g^0$, hence no unpaired electron and the complex is diamagnetic ($\mu=0$). The cation $[\mathrm{Co(NH_3)_4Cl_2}]^+$ is of type $\mathrm{[MA_4B_2]}$, which has two geometrical isomers, cis and trans. The trans isomer has several mirror planes, and the cis isomer also possesses a mirror plane (containing both Cl atoms and bisecting the two cis-NH$_3$ pairs), so neither is optically active. Option (D) wrongly assumes high-spin $d^6$ ($\mu=\sqrt{24}$ BM), (C) counts all chlorides, (B) confuses $[\mathrm{MA_4B_2}]$ with $[\mathrm{M(AA)_2B_2}]$.